1. Field of the Invention
The present invention relates to mirrors and more particularly to the protection of mirrors against corrosion, especially corrosion at the edges.
The mirrors to which the present description refers comprise in general a glass sheet having a reflective metal coating deposited on the glass surface and a protective coating applied to the reflective metal. Examples of commonly used reflective metals are silver and copper. The protective coating, which commonly comprises a layer of paint, serves in part to prevent abrasion of the reflective metal but more importantly provides the metal with resistance to corrosion. If such anti-corrosion protection is not provided the reflective metal tends to undergo oxidation or attack by atmospheric pollutants, resulting in tarnishing and discolouration and thus a reduction of the specular reflective properties of the mirror. The risk of corrosion is considerably increased if the mirror is used in humid conditions.
2. Description of the Related Art
Despite the presence of such a protective coating premature ageing of mirrors is often observed, either by the appearance of a haze, indicative of a slight oxidation of the metal, or by corrosion of the metal starting from the mirror edges.
No fully satisfactory means of resolving this problem has hitherto been proposed. In the case of a mirror with a copper layer, either used alone as a reflective layer or in combination with a silver layer, it has been proposed to coat the copper with an inhibitor based on an azole composition. Such proposals are for example made in British patent specifications Nos. 1 074 076 (Pittsburgh Plate Glass Co.), 1 250 142 (Shikoku Kasei Kogyo Co. Ltd) and 2 102 453 (Glaverbel) and in U.S. Pat. No. 4,255,214 (Falconer Plate Glass Corporation).
The use of azole-based inhibitors has given a discernible improvement in preventing or retarding the appearance of haze by hindering the oxidation of the copper, and consequently also of any underlying layer of silver. It has however been established that even when such inhibitors are employed the problem of corrosion at the margins of the mirror has not been fully solved and could therefore lead in time to an unacceptable reflective quality.